Why are my Bandit’s spark plugs black?

Q. I recently got a 1999 Suzuki Bandit GSF600S that was running great until about two weeks after I bought it.

There was no spark coming off them, so I went out and replaced all four plugs and the bike started working great again. I thought I had sorted the problem, but then a week later it stopped again.

When I looked at the plugs they were black. I've been told this means the mixture is too rich. Any ideas where I should start?
Bboypredator, MCN forums
 
A. It’s going to be the fuel tap, float levels or needles. The simplest one is the fuel tap; it comes with three lever positions, “on”, “res” and “pri”. It is a vacuum tap that relies on the lower inlet air pressure when the engine is running to draw fuel into carbs in the “on” or “res” positions.

The “pri” position is used to “prime” the carbs with fuel after servicing work, and should only be used then. If you leave it on “pri”, thinking that is off, you can flood the fuel system.

The other causes are incorrect float heights or wear in the needle seats, excessively richening up the fuel mixture. You could also check that the air filter isn’t blocked.